Proposal: Improved Wildcard Syntax for Java (original) (raw)

Reinier Zwitserloot reinier at zwitserloot.com
Sun Mar 1 10:49:06 PST 2009

Neal, what I mean is the following sample cannot be translated to your
'out/in' syntax:

 public static <T extends Comparable<? super T>> void sort(List<T>

list);

The "T extends Comparable" part cannot be replaced with just 'out
Comparable'. My suggestion involves allowing this to be rewritten as:

 public static <T out Comparable<in T>> void sort(List<T> list);

Though, I share Stephen Colebourne's concern that the out/in syntax
can be just as confusing in other situations, again highlighting how
out/in and extends/super would co-exist (instead of a slow but steady
migration towards out/in syntax), complicating the language.

--Reinier Zwitserloot

On Mar 1, 2009, at 08:28, Neal Gafter wrote:

On Sat, Feb 28, 2009 at 11:08 PM, Reinier Zwitserloot <reinier at zwitserloot.com> wrote:

This proposal is lacking in the 'disadvantages' section, in two ways.

The listed disadvantage is unfairly sugared over; because the 'out/ in' terminology does not allow you to bind the generics parameter, the 'extends'/'super' syntax will never become outdated and the need to remember that 'out' is analogous to 'extends' and 'in' is analogous to 'super' does not go away. I don't understand. out/in bind the generic parameter in precisely the same way that ?extends and ?super do. What you describe is a disadvantage of the current syntax, but when using the new syntax you don't have to remember it, because the new syntax is self-mneumonic. There's another disadvantage that isn't listed at all: Neither 'in' nor 'out' are keywords in java currently. Therefore, you introduce the notion of a context-sensitive keyword, but as far as I understand it, java does not currently have context-sensitive keywords. Adding them is a big can of worms that should probably warrant some discussion (in the sense that it may complicate java parsers, and how the idea of context-sensitive keywords makes it a lot harder for any parser to use a keyword as an anchor point); at any rate it is a disadvantage that should be listed. Any change in syntax has to be considered for its interaction with other tools, but for this particular case the general concerns you express don't seem to be an issue. For example, this syntax is very easy to parse with no more than the usual one-token lookahead. There's also a typo in 'mneumonic' :) NB: For what it's worth, because the need to remember super-produces extends-consumes does not go away, I don't think this is going to help much. You don't have to remember that when reading and writing the new syntax. Perhaps make 'out' and 'in' valid context-sensitive keywords that can be used as a substitution for super and extends That's exactly what we've done, but with in/out you don't use the question mark either. and add that a missing generics variable on the LHS implies a wildcard? I don't understand what you propose, or see how it relates to this proposal.

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